The invention is related generally to sensors and more particularly to fluid line sensors for monitoring the existence and contents of a fluid line.
Air-in-line sensing systems typically include a pair of transducers positioned opposite each other, between which is disposed a fluid line to be sensed. One of the sensors is used to transmit ultra-sonic acoustic energy and the other is used to receive the energy. The fluid line interacts with that transmitted acoustic energy and that interaction is detected by the reveiver transducer. The output of the receiver transducer can be processed to determine certain properties of the fluid in the sensed fluid line.
Such a sensor system has had an application in the medical field. Because an air bubble of sufficient size has the potential to cause death in a patient if infused, air-in-line sensor systems are routinely used to sense for the existence of air in intravenous infusion lines. The acoustic impedance of intravenous infusion solutions is readily discernable from the acoustic impedance of air. Air positioned between the transducers increases the impedance between those transducers and results in a reduction in the output of the receiver transducer. Upon detecting an air bubble of a particular size, an alarm is provided and the infusion pump operation is stopped.
One requirement of acoustic air-in-line sensor systems is that firm and as uniform as possible tubing-to-transducer contact be made so that only air bubbles residing inside the fluid line are detected. If poor contact exists, air may reside between the outside surface of the tubing and a transducer. The sensor will be unable to determine if this air is inside the tubing or outside. Air outside the tubing but in the sensing path will result in a decrease in the signal-to-noise ratio of the sensor system or worse, an air-in-line alarm. Because of this undesirable consequence, great effort has been applied to developing the geometries of transducer-to-tubing interfaces to obtain firm and uniform contact. In cases where the tubing is over-compressed in the sensor, it has been noticed that the contact pressure between the tubing and the transducer diminishes in some areas and may even cease to exist thereby allowing air between the exterior of the tubing and the sensor.
Some prior sensing systems comprise a channel into which the fluid line is pressed and comprise transducers located on opposite sides of the channel which face each other. In these systems, the same type of fluid line operated on by the associated controller or pumping mechanism is sensed by this air-in-line sensor system.
In the case where the controller or pump comprises a peristaltic mechanism, the fluid line is formed of a compliant material so that the peristaltic pumping mechanism can occlude the tubing along a pumping zone. Silicone or PVC tubing is typically used. If no strain relief is included between the pumping mechanism and the following air-in-line sensor, the mechanical action of the closer peristaltic fingers may cause a change in the dimensions of the tubing sensed by the air-in-line sensor and the performance of the air-in-line sensor may degrade. Such a result can occur where the air-in-line sensor is located so close to the last fingers of the pump that the fingers pull the tubing into a deformed shape during occlusion and this shape change extends into the air-in-line sensor segment of the tubing. Additionally, typical fluid tubing subjected to heat and the continuous contact pressure in an air-in-line sensor loses its resiliency and the contact pressure between it and the air-in-line sensor may diminish thereby degrading the performance of the sensor. Tubing manufacturing tolerances which vary significantly also pose difficulty in the design of the air-in-line sensor.
Another problem affecting prior air-in-line sensor systems is the presence of contamination adhering to the segment of tubing being sensed. Infusion fluids or other fluids or particulate matter reaching the outside of the fluid line either through handling of the fluid line or through leaking fluid couplings located above the sensed segment can affect the transmission of the acoustic energy between the transducers and may decrease the accuracy of air-in-line detection. It would be desirable to provide a sensing system which is resistant to external fluid line contamination.
A further problem affecting some air-in-line sensing systems is the interference with the sensing process caused by nuisance air bubbles which either become transfixed on the interior tubing wall at a position between the transducers, or move between different positions inside the tubing in the path of the sensing energy of the transducers. In many cases, an original transfixed air bubble alone is not large enough to cause an air-in-line alarm if detected but these bubbles tend to grow in size and after reaching a certain size, can significantly interfere with the sensor. Air bubbles which oscillate between positions in the sensing segment can also grow and their presence can lower the effectivity of the sensor.
It has been found that silicone tubing, which is commonly used as the pumping segment of the fluid line with peristaltic pumps, is permeable enough to allow air bubbles to enter the tubing which has an inside surface promoting attachment of the air bubbles to that surface. It has also been noted that such air bubbles tend to cling to the inside surface of PVC tubing as well. In the case where the fluid line is oriented vertically and low pump rates are used, such bubbles may move from a point upstream in the fluid line to a point downstream in the fluid line and then float back upstream again. It would be desirable to provide an air-in-line sensor system which resists the formation of nuisance bubbles.
Hence, those concerned with air-in-line sensor systems have recognized that it would be of value to provide a system which is not susceptible to degradation due to its placement next to the pumping mechanism. It would also be of value to provide a system which is not sensitive to tubing tolerance variations and which does not experience reduced performance when subjected to heat and continued contact pressure. It would also be of value to provide a system which resists the formation of transfixed or oscillatory air bubbles between the transducers. The present invention fulfills these needs.